The present invention relates generally to methods for increasing the efficiency of cross-flow retort processes for recovering shale oil from oil shale.
In cross-flow retort processes for the recovery of oil values from shale, a flow of hot gases is forced through a gas-permeable moving bed of crushed shale particles (typical operations using, for example, a traveling grate of the type used in iron ore pelletizing). The hot gases cause thermal removal of the oil from the shale and conversion of the oil into vapor phase constituents. As these vapors are carried by the downdraft through the lower temperature regions of the bed, which are below the varporization point of the bulk of the oil, the oil is condensed to a liquid phase, yielding a fog or mist of suspended oil droplets in the otherwise gaseous medium. Separation of these oil droplets from the gas medium and condensation of the otherwise vapor phase oil constituents is carried on outside the cross-flow apparatus. The entire heating zone is kept sealed from exposure to the atmosphere in order to prevent loss by oxidation and/or burning of recoverable oil components.
The moving bed then travels to a second zone, where it is cooled by a cross-flow of incoming cool gas to recover sensible heat from the solids. The off-gas from the cooling zone may then be further heated and utilized as part of a preheating or heating operation. In some applications, the off-gas from the heating zone is passed through the incoming solids bed prior to the heating zone in order to preheat the bed and maximize the recovery of available sensible heat from the gas.
Depending upon such things as the type of shale used, the particle size and thickness of the bed, and the temperatures, time sequence, and number of recycle stages carried out in the retort process, inter alia, the spent shale discharged from a typical retort apparatus contains anywhere from about 0.5 to about 4% or more by weight of a carbonaceous residue, which is generally non-volatile and is fixed on the surface and within the pores of the spent shale particles.
The exact chemical nature of this residue is largely unknown, it being most probably a combination of carbon and high molecular weight hydrocarbons. The potential energy producing value of such carbonaceous material represents typically from about 5 to about 30% of the total organic heating value of the shale.
Attempts at recovery of this carbonaceous residue would be worthwhile, if for no other reason than its organic heating value. Just as important, however, is the fact that the presence of this residue interferes substantially with the convenient and economic recovery of other mineral values (most importantly, aluminum) from the spent shale particles. For example, the high organic materials contained in such residue create problems in wetting the shale particle, produce unwanted foaming and also are a source of contamination of any recovered products. The carbonaceous residue thus must either be removed from the particles before further processing or the particles ground to a finer state in order to expose more surface area to the chemical processes which effect the removal of aluminum. In any event, removal of the organic materials in the aluminum recovery process presently requires additional expensive equipment.
It has been proposed in the prior art to recover the energy value of this residual carbonaceous material by outright combustion of the residue. The heat of combustion may then be utilized in providing heat to the initial stages of the retort process. However, the high temperatures inherent in such a procedure create their own problems.
For example, it is known that at temperatures of about 1600.degree. F, the softening point of some of the minerals contained in the shale ash is reached and sintering occurs, resulting in what are known as "clinkers." Needless to say, when sintering occurs, additional physical treatments are required in processing the shale ash for recovery of mineral values.
More critically, temperatures in excess of 1400.degree.-1500.degree. F produce chemical and physical changes in the shale ash which result in the aluminum and sodium values no longer being removable by convenient leaching techniques.
Finally, at about 1350.degree. F, a number of chemical reactions which are endothermic in nature begin to occur. Even where recovery of aluminum, magnesium or sodium by further processing of spent shale is not contemplated, these endothermic reactions still represent a considerable waste of energy.
Heretofore, it has not been apparent that oxidation of the carbonaceous residue on the spent shale particles could be controlled. The difficulty lies in providing a system which would balance the introduction of heat to the burden so that the bed is neither overheated nor over-cooled. The undesirable effects caused by overheating have been previously explained. A minimum temperature is also important since if the particles are cooled before essentially all of the carbonaceous material thereon is removed, the oxidation reaction is quenched when the surface at which the reaction is occurring falls below the ignition temperature.
It is a primary feature of the present invention that overall thermal efficiency of the retorting process is increased while at the same time economically feasible utilization of the total mineral content of oil shale can be realized.